The design of much equipment, such as pumps, compressors, and valves (and some miscellaneous pressure vessels) requires that a shaft, rod, plunger, or stem extend through the wall of the equipment. The internal cavity of the equipment is exposed to a pressure above or below atmospheric pressure. A sealing arrangement of some type is therefore required at the junction of the shaft and equipment wall to restrict the leakage of fluid from the equipment into the environment, or leakage from the environment into the equipment. This sealing arrangement is often installed in a cavity at the wall of the equipment, this cavity often being called a stuffing box. The sealing arrangement is usually in the form of rings of packing, or a mechanical seal. The packing takes many forms, including bulk fibers, fibers plaited or braided into a rope (and cut to fit the box and shaft), and molded or machined rings --often containing sealing lips.
The stuffing box of a reciprocating pump or compressor, because of the axial movement of the piston rod or plunger, is unsuitable for the application of a mechanical seal of the type used for a rotating shaft. Stuffing boxes of centrifugal and rotary units, because of internal or external environmental conditions, may also be unsuitable for mechanical seals. In such cases, rings of conventional or special packing are utilized to achieve a seal in the stuffing box.
FIG. 1A discloses a conventional stuffing box apparatus 10, including a housing 12 having therethrough a cylindrical opening or bore 13, having internal threads 16 at one end of the bore 13, and having a radially inwardly extending annular flange 17 at the opposite end of the bore 13. The housing 12 is sometimes referred to in the art as a "stuffing box", but the term "housing" is used herein to avoid confusion about whether a reference to "stuffing box" means the entire assembly or just the housing 12. Two spaced annular bushings 18 and 19 are disposed within the bore 13, and a stack 22 of four annular packing rings 26-29 is disposed within the bore 13 between the bushings 18 and 19. An annular gland 31 has external threads 32 which cooperate with the threads 16 on the housing 12, and rotation of the gland 31 relative to the housing effects variation of compressive forces exerted on the bushings 18 and 19 and stack 22 of packing rings by the gland 31 and flange 17. A cylindrical plunger or piston rod 34 extends axially movably through an opening 33 in the gland 31, and through each of the bushings 18 and 19 and each of the packing rings 26-29. The inside diameters of the packing rings 26-29 and bushings 18 and 19 are approximately equal to the outside diameter of the rod 34. A fluid pressure (which for purposes of this discussion is greater than atmospheric pressure) is present at the left end of the housing 12 and fluid attempts to flow rightwardly in FIG. 1A in the direction of the arrow 36 along the surface of the rod 34.
A graph shown at 39 in FIG. 1B has lines 41-45 which correspond to the various axial ends 53-57 of the packing rings 26-29, and depicts with a curve 48 approximately how the packing rings 26-29 reduce the fluid pressure from the elevated pressure Pd present at the left end of the housing 12 to atmospheric pressure Patm. As evident from the graph 39, the packing ring 26 produces very little pressure drop, the packing ring 27 produces only a slightly greater pressure drop, and the packing ring 28 produces only a slightly greater pressure drop, whereas the packing ring 29 produces significantly more than one-half of the required pressure drop. Consequently, the packing ring 29 is subject to undesirably rapid wear, and often tends to extrude axially rightwardly adjacent to the radially inner surface or the radially outer surface of the bushing 19.
To achieve long packing life, it is necessary to minimize extrusion of the packing. To minimize extrusion, it is necessary that the clearance at 51 between the rod 34 and internal diameter of the backup bushing 19 be small. Higher pressures require smaller clearances. If this clearance is too large for the combination of packing type and pressure, the packing will extrude through the clearance, which results in leakage and premature failure. In this conventional stuffing box apparatus, the clearance at 52 between the outside diameter of the backup bushing 19 and the bore 13 must also be kept small to prevent packing extrusion. Such construction fixes the location of the backup bushing laterally in the stuffing box, and the close clearances require that the rod 34 operate very near the center of the box. Such precise alignment is seldom obtained or maintained in industrial equipment. The inevitable misalignment causes the rod 34 to rub the inside surface of the backup bushing 19, increasing the clearance at 51 and often scoring the rod, both of which contribute to short packing life.
This conventional type of stuffing box apparatus 10 contains a single stack 22 of packing rings. The total differential pressure thus exists across the single stack 22 of rings, the pressure gradient being approximately as indicated at 48 in FIG. 1B. The ring 29 on the atmospheric side of the stack 22 experiences the maximum pressure differential, does most of the sealing, and experiences the greatest wear and extrusion. Some proprietary lip-type packing designs have resolved this problem by incorporating into the packing a hard center section which absorbs the axial force from adjacent rings without transmitting the force to the lips, but such designs are limited to lip-type rings, which find limited applications. This problem is also partially resolved by a stuffing box design which divides the packing into two separate, independently-supported stacks, although such designs are special and require more axial space.
A third known design, used extensively on reciprocating compressors, provides a segmented stuffing box with hard segmental rings. This design does provide for independent support of individual sets of packing, and allows for more radial misalignment, but a special box and packing are required, and a fairly high degree of skill is required for maintenance. Application to pumps has been successful in only a small range of services.
As a result, there has been a need for a sealing device which would allow operation at higher pressures and/or with longer life, that would be installable, with minimal skill, in a conventional stuffing box, and that would allow for misalignment of the rod with the stuffing box.